This invention relates to breaking an emulsion formed when a predominantly 1,2-dichloroethane (ethylene dichloride, "EDC") stream containing at least 15% by weight (wt) of highboiling chlorinated hydrocarbons (also referred to as highboils, or "CHC" for brevity), and contaminated with iron salts, is acidified with aqueous hydrochloric acid. If the emulsion is not broken, the aqueous and organic phases cannot be separated by gravity, allowing the mixture of phases to settle. Since other methods of phase separation of such a mixture are not economical, the ability to break the emulsion and make the separation is critical in a commercially viable process. This invention is directed to demulsifying an acidified bottoms draw-off from a high temperature chlorination (HTC) reactor, which draw-off is predominantly EDC containing about 2000 ppm to about 4000 ppm of Fe present as a salt, and at least 15% by wt highboils, then separating the aqueous and organic phases. The organic phase contains less than 10 ppm Fe which is concentrated in an organic waste stream to be incinerated by contact with a fluid-bed catalyst without poisoning it. The catalyst is held in a catalytic oxidation ("Catoxid") reactor in a commercial plant for the production of more than 600 million pounds per year of vinyl chloride.
The "direct chlorination of ethylene" is the basis for the widely used commercial catalytic process for the production of EDC. The reaction is controlled by mass transfer, with absorption of ethylene as the limiting factor whether the reaction is carried out with a slight excess of ethylene, or as an alternative option, a slight excess of chlorine, fed to the reactor. The heat of reaction is dissipated either through conventional water cooling of a typical low temperature chlorination ("LTC") also referred to as a "non-boiling" reactor because it operates in the range from about 50.degree. C. to about 65.degree. C.; or, by operating the reactor at, or near, the boiling point of EDC under superatmospheric pressure up to about 25 psig, preferably from about 5 psig to 25 psig, hence referred to as a HTC reactor, also referred to as a "boiling reactor" when it is operated at the boiling point of EDC.
The direct chlorination reaction may be written: EQU CH.sub.2 .dbd.CH.sub.2 +Cl.sub.2 .fwdarw.ClCH.sub.2 CH.sub.2 Cl
and theoretically, neither water nor HCl is formed as a product of this reaction. In practice, in the presence of oxygen, some water may be formed in some side reactions, and some HCl is formed in another side reaction which may be written: EQU ClCH.sub.2 CH.sub.2 Cl+Cl.sub.2 .fwdarw.ClCH.sub.2 CHCl.sub.2 +HCl
The precise amount of HCl formed depends upon the type of catalyst used in the HTC reactor, upon the liquid medium in which the reaction is carried out (typically a chlorinated hydrocarbon such as EDC), and upon the conditions of reaction.
The direct chlorination reaction is carried out with a variety of iron-containing catalysts such as ferric chloride, sodium or potassium tetrachloroferrate, and ammonium tetrachloroferrate which must be maintained in a specified concentration in the reaction mass. During the reaction, numerous chlorinated byproducts are formed which must be removed to avoid stifling the efficient formation of the desired EDC. Of course, it would be best if the byproducts could be removed without removing the catalyst, but since this is not possible, the catalyst is depleted when the byproducts are purged from the reactor.
The draw-off is predominantly EDC, which may be present in the range from about 50% to about 98% by wt of the liquid reaction medium in the HTC reactor, typically from 60 to 95% by wt, the remainder being "heavies". By "heavies" we refer to polychlorinated CHC which are higher boiling than EDC. These heavies include polymeric materials some of which are solid or semi-solid. Economics dictate that the EDC in the draw-off be recovered in a distillation column for the heavies ("heavies column"). The problem is that the presence of FeCl.sub.3 in the bottoms of the heavies column results in agglomerates which are caked onto the internals of the column, the surfaces of the tubes in the column's reboiler, the valves, etc., requiring frequent shut-downs to clean the equipment.
Further, if the draw-off from the bottom of the heavies column ("heavies" draw-off) is to be disposed of by combustion in a catalytic oxidation reactor using a catalyst which is progressively deactivated by the presence of metals, particularly iron, sodium, potassium and the like, the metals content of the feed to the oxidation reactor must be decreased to a level at which the deactivation of the oxidation catalyst is tolerable.
This invention relates particularly to the use of such salts as the tetrachloroferrates and FeCl.sub.3 which must be removed from the bottoms draw-off of the HTC reactor if the draw-off is to be concentrated, and the concentrate oxidized in a specific type of catalytic oxidation reactor, namely the "Catoxid" reactor. The Catoxid reactor uses a gamma alumina catalyst which is deactivated by contact with metals such as iron, sodium and potassium even when they are present in minimal concentration. The heavies draw-off is typically neutralized with ammonium hydroxide or ammonia before being fed to the Catoxid reactor. When the neutralized and concentrated draw-off contains more than 50 ppm Fe present as FeCl.sub.3 or a ferrate salt, the catalyst is deactivated in an unacceptably short period of time. Thus more than 50 ppm Fe in a wastestream fed to the Catoxid reactor is said to be toxic to the oxidation catalyst, and the catalyst is said to be adversely sensitive to the presence of metals, because it is uneconomical to replenish the deactivated catalyst.
The iron concentration (computed as ppm of Fe) in the bottoms draw-off is tied to the concentration of FeCl.sub.3 after acidification of the draw-off, because it has been found that, when the operation of the heavies column and/or tar still, the HTC reactor, and the Catoxid reactor are combined under carefully regulated conditions, a concentration of from 2000 ppm to 4000 ppm Fe (which may be present as FeCl.sub.3 or a tetrachloroferrate salt) in the HTC bottoms draw-off essentially eliminates its emulsification provided (1) the draw-off is contacted with an aqueous hydrochloric acid having a concentration of from 2% to 4% by wt; (2) the draw-off is mixed with at least a fifty-fold excess of the aqueous HCl acid over the amount of HTC bottoms draw-off; and, (3) only the draw-off from a LTC reactor, if one is operated in the plant, and no other chlorocarbon stream containing Fe contaminants, may be added to acidified HTC bottoms before it is demulsified and separated into aqueous and organic phases.
It will be evident that the concentration of catalyst in the HTC reactor must be replenished so as to result in a concentration of from 2000 ppm to about 4000 ppm Fe in the bottoms draw-off stream from the HTC reactor before it is acidified with aqueous hydrochloric acid. It is equally evident that, since the goal of the detoxification process is to provide a draw-off from the heavies column and/or a tar still with less than 50 ppm Fe, and the waste heavies from the HTC reactor eventually finds its way to the heavies column, it would seem desirable to minimize the concentration of Fe in the HTC bottoms, not deliberately run it in the 2000-4000 ppm range.
The rate of withdrawal of the bottoms draw-off from the HTC reactor is such as to control the build-up (concentration) of heavy, high-boiling chlorinated byproducts in the HTC reactor. EDC in the HTC bottoms draw-off must be recovered in the overhead of the heavies column, and the remaining heavies bottoms further stripped of EDC and desirable CHC, then disposed off by incineration in a fluid bed oxidation reactor. The particular make-up of the bottoms draw-off with particular respect to the emulsionforming CHC solids and polymers, depends upon the concentration of chlorination catalyst used in the HTC reactor and its process operating conditions.
In the particular commercial process of interest herein, the bottoms draw-off contains in excess of 60%, preferably about 80%, by weight EDC, and also about 15% by wt of higher-boiling (than EDC) liquid CHC byproducts ("highboils") including a semi-solid mass of polychlorinated CHC and other unidentified relatively high molecular weight compounds collectively referred to as "tar" because it is an unfiltrable muck. By "unfiltrable" is meant that it can be filtered through a basket filter having a coarse mesh size larger than about 0.0625 inch (1.59 mm), say about 12 mesh U.S. Standard), but it is impractical to filter the tar through a smaller mesh size than 177 microns (80 mesh) on a commercial scale, even if such filtration removed the emulsifiers in the stream, which, once the emulsion is formed, by definition, filtration cannot. Filtration of the HTC bottoms stream is impractical even after the addition of a filter aid, because the filter is blinded in less than an hour. Filtration through a 105 micron filter (140 mesh) blinds the screen almost immediately even when the organic stream is diluted with 10 volumes of dilute HCl acid.
It is this HCl acid-diluted tarry two-phase emulsion of EDC, heavies and aqueous ferric chloride, which is to be separated into aqueous and organic phases by a simple decantation step. Formation of an emulsion does not permit separation of the aqueous and organic phases by decantation because a supernatant aqueous layer is not formed. For such separation, when an emulsion is formed, the emulsion must be broken or the phases will not separate in a phase separation drum.
The aqueous phase is led to waste water treatment. The separated organic phase (hereafter "separated waste organic stream" because it is phase separated) is neutralized with alkali, usually caustic soda, ammonia or ammonium hydroxide, to convert the ferric chloride to ferric hydroxide, and the neutralized separated waste stream is led to a light ends column. The bottoms from the light ends column is led to a heavies column, then preferably to a tar still, for recovery of the EDC.
If the Fe concentration in the bottoms draw-off of the HTC reactor is sufficiently high to be of concern, the obvious solution to the problem is to acidify the draw-off stream with dilute HCl, but this causes such severe emulsification that the emulsion cannot be broken using prior art techniques on a commercial scale, and the organic and aqueous phases cannot be separated by settling. It is this critical problem of how to demulsify the acidified CHC, and make the separation of aqueous acid and organic phases, which has never been addressed in the prior art with sufficient detail as to provide the essential critical teachings to enable one skilled in the art to solve the problem. This deficiency is particularly conspicuous because it is well recognized that the behavior of different emulsion systems is highly specific, so that it is unrealistic to expect that a system can be demulsified without specific teachings as to how this should be done.
We are aware of some general tenets of methods for demulsifying a stream, including (a) flocculating the stream to such an extent that coalescence of the droplets is no longer prevented by the interfacial film, (b) removing the emulsifying agent from the interface, and (c) forming an emulsion of the opposite type. We are also aware of the many ways to accomplish the foreging methods, including special mechanical treatments, heat treatments, electrical deposition, freezing, exposure to supersonic vibrations, filtration through a filter medium which is preferentially wetted by the discontinuous phase of the emulsion, and addition of surface active agents or other special chemicals. We have been unable to break the emulsion we form with prior art processes, try though we may, and we know of no one who has done so if only on a bench scale in a laboratory.
Such a prior art process is disclosed in U.S. Pat. No. 4,533,473 to Burks, Jr. et al, assigned to Stauffer Chemical Company which discloses the same basic problems with the separation of FeCl.sub.3, and the same process operations as in our plant for the production of EDC, except for the difference in how the waste CHC streams are treated. In the '473 process, several waste CHC streams are combined and concentrated in a tar still or by vacuum distillation to produce a residual product ("residue") of highboils, as was conventionally done, and still is, in the prior art. This residue includes EDC, 1,1-dichloroethane, dichloroethylenes, trichloroethylene, perchloroethylene, pentachloroethylene, 1,1,2-trichloroethane (triane), chloroform, 1,1,1-trichloroethane (methylchloroform), 1,1,2,2-tetrachloroethane; penta- and hexachloroethanes, and chlorobutadienes such as chloroprene. Most of all, combining waste streams produces a combination of emulsifiers which produce so stable an acidified aqueous emulsion that, despite affirmations and allegations to the contrary, a combination of several waste CHC streams cannot be demulsified, then separated in a phase separation drum.
The '473 process failed to recognize that the emulsification effect of emulsifiers from the heavies CHC stream from the HTC bottoms in particular, was so intense that the emulsion formed upon acidification defied separation except under narrowly defined conditions, particularly that the concentration of dilute HCl acid for demulsification was critical, and that the amount of dilute acid required for doing so was orders of magnitude greater than what was estimated to be sufficient. The problem of dealing with the emulsion formed with the HTC bottoms containing ferric salts, was simply overlooked, as evidenced by the statement "depending upon the nature of the waste stream and the metallic contaminant, an emulsion may be formed which may easily be broken." (see col 4, lines 40-43). This oversight is re- affirmed in the statement "Even should an emulsion form, it has been found that treatment according to this invention creates emulsions which may readily be broken by passing the treated material through a filter and coalescer, optionally with the addition of a filter aid or by centrifugation (sic)." (see col 5, lines 24-28).
The '473 patent particularly sets out to teach how to detoxify any CHC stream, before or after concentration, or, any one or more of combined waste streams from (i) a HTC reactor in a process for making EDC, and (ii) other processes for the production of chlorinated hydrocarbons such as chlorinated benzenes and various chloromethanes, described and schematically illustrated in FIGS. 1-4 of the reference. The process, schematically illustrated in FIG. 5, teaches first filtering the waste residue(s) from one or more of the foregoing chlorination processes to remove particulate solids, then contacting the filtrate of the residue with a dilute aqueous solution of a mineral acid, then coalescing, filtering or centrifuging the acidified residue "if necessary, to break any emulsion which may have formed" (see col 9, lines 44-45). Not only is there little weight accorded the seriousness of the emulsification problem, but a broad range of aqueous acid is said to be effective. It is stated that the dilute aqueous solution contains preferably from 2 to about 10 percent by wt of acid, the volume ratio of dilute acid to organic material in the waste stream being at least 1:1 to dissolve the FeCl.sub.3 in the residue.
If the emulsion had been successfully demulsified, it would have been evident that breaking the emulsion to make the separation was the overriding consideration to enable one to carry out the process. The economic desirability of combining several waste heavies CHC streams for Fe removal would have been unequivocally negated by the exigent requirements for breaking the emulsion.
Our attempts to remove slightly higher than 50 ppm Fe concentration by acidification with aqueous HCl were unsuccessful because of the formation of the emulsion which defied partition into the aqueous and organic phases economically. It was therefore quite surprising that a much higher concentration, namely 2000-4000 ppm of Fe in HTC bottoms, appears to generate emulsifiers which form an emulsion amenable to being demulsified, and is the key to a successful separation.
We find that, either before or after we acidify a draw-off which contains 2000-4000 ppm Fe, the draw-off cannot be filtered through a screen smaller than 80 mesh (U.S. Standard). We find that any attempt to filter out small solids such as may initiate formation of the emulsion, leads to blinding the filter. We have found no compound to coalesce the emulsion, and no adsorbent to adsorb the contaminants in any practical fashion. The sole purpose of a basket filter is to remove large lumps of solids and heavy, creamy semi-solid agglomerates referred to as "rags" (because of their appearance), to minimize fouling of the equipment.
The operating difficulty of dealing with the HTC bottoms stream was never properly appreciated even in U.S. Pat. No. 4,614,643 to Doane, (co-inventor with Burks, Jr., filed much later than the '473 reference and also assigned to Stauffer), which suggests a tarry bottoms stream from a still can be treated with water and either filtered or centrifuged. More particularly, the '643 patent discloses mixing a wide variety of FeCl.sub.3 -containing chlorocarbon streams, treating them in an intermediate processing section in which the products are treated using any one or more "unit operation" steps, then distilling the treated stream in a still to recover valuable chlorocarbons. The bottoms stream from the still contained the FeCl.sub.3 to be removed. Just enough water is added to this bottoms stream to convert the FeCl.sub.3 to FeCl.sub.3.6H.sub.2 O (hexahydrate) which is then removed either by filtering or centrifuging the stream. The operability of the process depends upon the presence of the solid FeCl.sub.3.6H.sub.2 O which can be centrifuged because even the diluted waste stream is unflitrable.
The '643 patent acknowledges that German patent No. 2,540,292 states that an ordinary washing of EDC with water is not generally satisfactory for removing FeCl.sub.3, therefore uses a multi-stage side channel pump. The '473 and '643 patents both refer to the teachings of Japanese Patent Publication No. 13606/1966 which discloses that dilute acid is preferred over water for washing a product EDC stream because of the emulsification problem (when using water). Note however, that product EDC, even if it is the bottoms from a LTC reactor, has much less than 100 ppm Fe; if the EDC is the overhead from the HTC column, or any other column in the process train, the EDC contains less than 10 ppm Fe, more typically less than 1 ppm.
In no instance does the EDC from any of the foregoing sources generate an emulsion when it is acidified with dilute HCl, irrespective of the acid concentration. Clearly there is nothing in the obvious treatment of a Fe-containing EDC stream with dilute HCl acid, to enable one to discover that within a particular range of elemental Fe concentration, a predominantly EDC bottoms stream containing at least 15% CHC heavies and tar, would generate such an intractable emulsification problem, nor that this problem could be solved if the emulsion was thoroughly mixed with at least a 50-fold excess of dil HCl in the 2%-4% range.
The numerous prior art references referred to in the '473 and '643 patents uniformly deal with an essentially EDC stream contaminated with an unspecified, and when specified, small amount of FeCl.sub.3, not a concentration in the critical range of from 2000-4000 ppm Fe; nor do they deal with EDC contaminated with a large proportion of highboils and solids. None of the many prior art references suggests what we discovered, namely how disabling the emulsification problem really is with such a high Fe concentration in EDC containing at least 15% by wt CHC highboils and tarry solids.
It is evident that, if iron is precipitated as ferric hydroxide by distillation in a heavies column, the precipitated ferric hydroxide could not be removed from the feed to the Catoxid reactor, and the ferric hydroxide cannot be, because it would leave the bottoms of the heavies column with the heavies to be combusted. In an essentially EDC stream, contaminated with 1000 ppm FeCl.sub.3 and less than 5 percent by wt highboils, neither adsorption nor distillation and subsequent filtration is an operating problem. As long as the concentration of FeCl.sub.3 in the bottoms draw-off from the HTC reactor is outside the range of from 2000 ppm to 4000 ppm Fe, neither the physical and chemical structure of the highboils and tar, nor the amount in which they are formed are such that formation of the resulting emulsion is a critical debilitating factor.
A technical appreciation of just how intractable a problem results from treating chlorocarbons with an aqueous acid solution is detailed in U.S. Pat. Nos. 4,307,261 and 4,412,086 to Beard et al who state "The most obvious method of removing ferric chloride from chlorinated hydrocarbon streams is the extracton of the ferric chloride with aqueous acid solutions. The ferric chloride is unexpectedly difficult to remove in this manner. Part of the ferric chloride apparently retains some solubility in the organic layer by forming complexes with polymeric material. Furthermore, the resulting chlorinated hydrocarbon product must be dried, which is an expensive procedure on an industrial scale." (see the '261 patent, 1, lines 27-36; the '086 patent, col 1, lines 31-40). In other words, extracting ferric chloride from any CHC stream with dilute acid would be obvious if it were not for the emulsification problem. We agree, except that we do not "dry" the product to burn it in the Catoxid reactor.
Our invention avoids filtering the bottoms draw-off stream, except for gross tarry material which can be removed in a basket filter with a mesh size large enough to avoid being blinded, allows separation of the unfltrable acidified organic and aqueous phases by decantation because no emulsion is formed, and permits directly flowing a non-toxic, neutralized but slightly alkaline acidtreated organic phase to a heavies column for recovery of EDC, and permits the bottoms from the heavies column or a tar still to be incinerated in a Catoxid reactor, thus providing a simple and practical solution to a difficult problem.